Fibroblast Growth Factor signaling in heart injury and repair Summary In response to injury, as in the case of cardiovascular disease (CVD), cells and tissues often reactivate developmental signaling pathways to repair damage or regenerate tissue. This can result in both beneficial and harmful consequences. To manipulate injury response mechanisms in a beneficial way, it is essential to understand the normal developmental pathways and how they are reactivated in adult tissues. In this proposal, we focus on Fibroblast Growth Factors (Fgfs), which are required for embryonic heart development, neonatal myocardial growth and the response to myocardial injury. Developmental and pathological studies demonstrate important interactions between cardiomyocytes, epicardial cells and cells derived from the epicardium, and vascular components of the heart. Recent evidence suggests that FGF signaling pathways regulate all three components of the heart during development and during the response to injury. Loss of FGF signaling in either epicardial or myocardial tissues impairs normal development and results in decreased heart size. Following injury to the heart in both humans and mice, FGF signaling increases, indicating that FGF signaling pathways may be necessary for cardiac repair. Following myocardial infarction, hearts lacking Fgf2 show reduced fibroblast proliferation and collagen deposition, decreased endothelial proliferation and vascular density, decreased cardiomyocyte hypertrophy, increased final infarct size (infarct expansion), and impaired cardiac function. Transgenic mice that overexpress Fgf2 in cardiomyocytes have decreased infarct sizes and increased recovery of function compared to WT hearts. These studies indicate an essential role for endogenous FGF2 in the heart's response to ischemic injury. Here we will explore the molecular, cellular and physiological mechanisms by which FGF signaling regulates interactions between myocardial, epicardial-derived and vascular components of the heart during homeostasis, physiological stress and response to injury (myocardial infarction). We propose to: 1) Identify the cellular targets of FGF2 required for cardioprotection; 2) Test the hypothesis that FGF9 and FGF16 function in vivo in the adult heart to limit FGF activity; 3) Identify molecular mechanisms that regulate cardiomyocyte responsiveness to FGFs in neonatal, adult and injured cardiomyocytes. These studies will provide essential new information about CVDs that are most likely to respond to FGF- based therapy, cell types that would be expected to show a beneficial (or detrimental) response to FGFs, and the mechanisms that regulate FGF activity in the context of potential endogenous inhibitors. This knowledge will be essential to effectively design and implement FGF-based therapies for the treatment of human CVD.